The Hawaiian Islands host a variety of rodents found from coastal areas to high-elevation forests. Understanding the specific types of rodents present, their characteristics, and how they arrived is the first step in recognizing their ecological role in this unique environment.
The Non-Native Status of Hawaii’s Rodents
The entire population of rodents in Hawaii consists of species introduced by humans. The Hawaiian Islands naturally evolved with only one native terrestrial mammal, the Hawaiian hoary bat (Lasiurus cinereus semotus). This lack of native ground-dwelling mammals meant the local flora and fauna had no defenses against the predation and competition introduced by rodents.
The timeline of their arrival is linked to human migration and trade. The first species, the Polynesian rat, traveled alongside the earliest Polynesian settlers centuries ago. Later arrivals—the black rat, brown rat, and house mouse—came primarily as stowaways on Western ships beginning in the late 18th and early 19th centuries. These introduced species are considered invasive, causing substantial damage to native ecosystems, contributing to the decline of endemic bird species, and altering plant communities.
The Three Rat Species
Hawaii is home to three species of rats, differentiated by size, physical features, and preferred habitat. The first to arrive was the Polynesian rat, followed by two larger species that arrived with Western contact. These three species, along with the common house mouse, make up the complement of rodents on the islands.
The Polynesian Rat (Rattus exulans)
The Polynesian rat, known in Hawaiian as the ‘iole, is the smallest of the three rat species. It has a slender body, a pointed snout, and large ears, reaching a head-and-body length of 4.5 to 6 inches. A distinguishing feature is that its tail is roughly the same length as its head and body combined, and it has a dark upper edge on the hind foot near the ankle.
This rat arrived with the earliest Polynesian voyagers, likely around 1000 AD, making it the first rodent to colonize the islands. Unlike the later-arriving rats, the Polynesian rat is commonly found in wild lowland habitats, such as agricultural areas and grassy gulches, and is less frequently a pest inside modern buildings. The species is an adept climber, often nesting in trees. Its diet includes seeds, fruits, and insects, making it a threat to native plant regeneration and bird eggs.
The Black Rat or Roof Rat (Rattus rattus)
The black rat, also known as the roof rat or ship rat, is a medium-sized rat, slightly larger than the Polynesian rat. Its body is slender, and its tail is nearly always longer than its head and body combined. The black rat is an excellent climber, preferring arboreal environments like trees, attics, and the upper parts of buildings.
This species arrived with Western ships beginning in the late 18th century. It is now the most common rat found in many wet and mesic native forests. Its arboreal nature makes it a threat to native forest birds, as it easily accesses nests in the tree canopy. The black rat’s diet is opportunistic, including a high percentage of fruit, and it is also known to prey on eggs and nestlings.
The Brown Rat or Norway Rat (Rattus norvegicus)
The brown rat, also known as the Norway rat or sewer rat, is the largest introduced rat species. Adults weigh between 10 to 18 ounces and reach 8 to 10 inches in body length. Unlike the black rat, the brown rat has a stout body, a blunt nose, and its tail is typically shorter than its head and body combined. It arrived around the same period as the black rat, establishing itself by the early 19th century.
The brown rat is a burrowing species that favors ground-level habitats, such as underground sewer systems, port areas, stream banks, and under concrete slabs. This species is a poor climber compared to the Polynesian and black rats, and is less commonly found in the native forest canopy. It is often associated with human habitation, creating extensive underground nest systems.
The Common House Mouse
The common house mouse (Mus musculus) is the fourth widespread rodent species in the Hawaiian Islands. It is significantly smaller than any rat species, with a body length of 3 to 4 inches and an equally long tail. The house mouse is characterized by its pointed snout, large rounded ears, and fur ranging from gray to light brown.
This rodent is a highly adaptable, commensal species that thrives in close association with humans, established in Hawaii by the early 19th century. While often found inside homes, wall voids, and commercial buildings, the house mouse is also distributed in wild habitats, including agricultural fields and high-elevation forests. They are prolific breeders and opportunistic feeders, contaminating food and causing damage by gnawing on various materials.